Stretch films are widely used in a variety of bundling and packaging applications. The term “stretch film” indicates films capable of stretching and applying a bundling force, and includes films stretched at the time of application as well as “pre-stretched” films, i.e., films which are provided in a pre-stretched form for use without additional stretching. Stretch films can be monolayer films or multilayer films, and can include cling-enhancing additives such as tackifiers, and non-cling or slip additives, as desired, to tailor the slip/cling properties of the film. Typical polymers used in the cling layer of conventional stretch films include, for example, ethylene vinyl acetate, ethylene methyl acrylate, and very low density polyethylenes having a density of less than about 0.912 g/cm3.
It is desirable to maximize the degree to which a stretch film is stretched, as expressed by the percent of elongation of the stretched film relative to the unstretched film, and termed the “stretch ratio.” At relatively larger stretch ratios, the film imparts greater holding force. Further, films which can be used at larger stretch ratios with adequate holding force and film strength offer economic advantages, since less film is required for packaging or bundling.
FIG. 1 shows an idealized tensile stress versus elongation curve 10 for a hypothetical stretch film. Curve 10 includes a first yield point 12, a second yield point 14, a natural draw ratio point 16, and a break point 18. Vertical line A passes through the first yield point 12, and indicates the elongation at first yield; horizontal line A′ passes through the first yield point 12, and indicates the tensile stress at first yield. Vertical line B passes through the second yield point 14, and indicates the elongation at second yield; horizontal line B′ passes through the second yield point 14, and indicates the tensile stress at second yield. Vertical line C passes through the natural draw ratio point 16, and indicates the elongation at the natural draw ratio point, this elongation value hereinafter termed simply the “natural draw ratio”; horizontal line C′ passes through the natural draw ratio point 16, and indicates the tensile stress at the natural draw ratio point. Vertical line D passes through the break point 18, and indicates the elongation at break; horizontal line D′ passes through the break point 18, and indicates the tensile stress at break. Region 20 of the curve, i.e., the region between the second yield point 14 and the natural draw ratio point 16, is termed the “yield plateau” region. Region 10 of the curve, i.e., the region between the natural draw ratio point 16 and the break point 18, is termed the “strain hardening region”. While these regions and features are shown in idealized form for a hypothetical film, it should be appreciated that in an actual film the stress-elongation curve has a continuous first derivative.
Several properties are desired in a stretch film. The tensile stress of the yield plateau, as characterized by the tensile stress at the second yield point 14 and at the natural draw ratio 16, correlates to the holding force the film can apply when stretched and wrapped around an article or a bundle of articles. Thus, it is desirable to have a large tensile stress at second yield and a large tensile stress at the natural draw ratio. The slope of the yield plateau 20 corresponds to the change in holding force as elongation increases, and so must be non-negative to avoid film failure. In a film with a positive, near-zero slope, as the film is stretched a small decrease in the film thickness due to small fluctuations in thickness uniformity can result in a large fluctuation in elongation, giving rise to bands of weaker and more elongated film transverse to the direction of stretching, a defect known as “tiger striping”. Thus, it is desirable to have a yield plateau slope large enough to avoid tiger striping over typical thickness variations of, for example, ±5%. For robust operation over a wide range of elongation, and using a wide variety of stretching apparatus, it is desirable to have a broad yield plateau region. In addition, since the extent of elongation correlates inversely with the amount of film that must be used to bundle an article, it is desirable for the film to be stretchable to a large elongation. While in principle the elongation at break is the maximum possible elongation, in practice, the natural draw ratio is a better measure of maximum elongation. Thus, it is desirable to have a large natural draw ratio. Other desirable properties, not illustrated in a stress-elongation curve, include high cling force and good puncture resistance.
While prior efforts have resulted in films having improved performance in one or several of the above-described properties, known films have not successfully displayed the combination of a large natural draw ratio, a large tensile stress at second yield and at the natural draw ratio, and a positive yield plateau slope large enough to absorb typical variations in film thickness uniformity without tiger striping.